A Low-Profile Vertically Polarized Patch Antenna With Wideband Omnidirectional Radiation Based on Triple Resonant Modes

In this letter, a low-profile, center-fed circular microstrip patch antenna (CMPA) is proposed. Under the concurrent operation of TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub>, TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub>, and TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">03</sub> resonant modes of a single CMPA, the vertically polarized (VP) omnidirectional radiation can be achieved in a wide bandwidth. Initially, the radiation mechanism and the far-field calculation of the TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0n</sub> mode of such a CMPA are investigated to demonstrate that the undesired radiation nulls of the higher modes can be removed by reducing the electrical length at resonance. To this end, a group of transverse slots is etched on the patch to lower the resonant frequency (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">03</sub>) and thus remove the radiation nulls from the TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">03</sub> mode. After that, two groups of shorting pins are introduced to raise the resonant frequencies of TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub>) and TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> mode (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub>) in proximity to f03, respectively. By combining the three resonant modes, the final design offers a wide bandwidth of 2.9 GHz to 3.9 GHz (29.4% in fraction) and a low profile of 0.034 times one free-space wavelength. Measurements evidently confirm its high efficiency, low cross polarization and stable VP omnidirectional radiation pattern over the entire operating band, making this antenna suitable for applications with full signal coverage in the 5G N78 band.